The big idea: Capillaries are where the blood exchanges materials with the cells. Every feature of them makes that exchange faster.
The points to remember
- Capillaries branch into huge networks: a large surface area for exchange.
- Their diameter is narrow, about one red blood cell, so they reach close to every cell.
- The wall is one cell thick (flat endothelial cells): a short diffusion distance.
- Oxygen and glucose leave the blood; carbon dioxide and wastes enter it.
Remember it as: Branched, narrow, thin: area, closeness, short distance.
Real example: skeletal muscle. Muscle is packed with capillaries: a working muscle fibre always has several running beside it, so oxygen and glucose only have a few micrometres to travel.
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Where exchange must be fastest: Some capillaries have fenestrations: small pores in the wall that let fluid and dissolved substances out and in very quickly.
| Where | Why fenestrated capillaries are needed |
|---|---|
| Kidney (glomerulus) | Huge volumes of plasma are filtered every minute |
| Villi of the small intestine | Digested food is absorbed into the blood fast |
| Glands that release hormones | Hormones pass into the blood quickly |
The points to remember
- On a drawing, an arrow for oxygen or glucose goes from the capillary into the cell.
- An arrow for carbon dioxide or wastes goes from the cell into the capillary.
- Capillaries are too small to see at low power: a red blood cell barely fits inside one.
Two traps: Write that the wall is one cell thick, not that it is 'a thin membrane': the wall is made of cells.
An arrow must go through the capillary wall to or from the cell, never straight from an artery to the cell.
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How it is asked: Expect 'describe or outline how capillaries are adapted for exchange', a one-mark 'state the adaptation', or an arrow to draw on a tissue diagram.
IB-style questionDescribe[2 marks]
Describe how capillaries in a working muscle are adapted for the exchange of glucose.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.